142 bulletin: museum of comparative zoology. 



the characters of glacial till, and I suggested that it was tillite. We 

 visited another locality at the junction of Ware and Centre Streets in 

 Roslindale where Dr. La Forge had found the same characters in the 

 rock, and this time were accompanied by Profs. W. M. Davis and 

 J. B. Woodworth. All agreed that this rock looked very much like 

 tillite. We visited the Hyde Park locality, and Professors Davis and 

 Woodworth were strongly of the opinion that the rock was tillite, 

 though the proof was still wanting. 



Criteria for the determination of till or tillite. 



As all till varies a great deal in its character in different localities 

 and in the neighboring exposures of the same locality, it is difficult or 

 impossible to give a short definition of it. Strictly speaking, the only 

 characteristic common to all tills and tillites is, that they are made up 

 of a miscellaneous, unsorted, unstratified mass of rock-debris laid 

 down by a glacier. As a rule till is composed of rock-flour or clay and 

 rock fragments, and in this mixture the matrix of rock-flour or clay 

 is a large part of the mass. The matrix, though more often of clay, 

 is sometimes sand of varying texture. 



Usually a majority of the stones in till are angular or bluntly angu- 

 lar (subangular), with several facets. Many have an oblong shape 

 with one or more flat sides or soles, smoothed or polished. Some of 

 the stones in till are rounded, water-worn pebbles, but these are usu- 

 ally in the minority. Where a glacier has pushed over gravel-beds, 

 many water-worn pebbles become included in the till without being 

 fractured or scratched. In southern Rhode Island the writer has seen 

 till made up almost entirely of rounded pebbles. In this case the 

 glacier retreated; the water from the melting ice deposited materials 

 at its front; the glacier then advanced over these deposits, which had 

 in the meantime become water-worn, making of them a mass which 

 can receive no other name than till. Here pebbles more than equalled 

 the sandy matrix in amount. 



Scratches or striae on the stones in the till are considered of prime 

 importance, in the proof of the glacial origin of a miscellaneous, un- 

 sorted, and unstratified mass like till. There is no doubt that striae 

 are most important, but they are not always conclusive proof of 

 glacial origin where present, and on the other hand the absence of 

 them is not conclusive proof of the non-glacial origin of any till-like 

 deposit. There are several mechanical agencies in nature capable 

 of producing striae besides glaciers; and over some areas in many parts 



